1. Field of the Invention
The present invention relates to image scanning apparatuses, image scanning methods, and computer programs. More particularly, the present invention relates to an image scanning apparatus that converts an image of a document part of which is corrected with correction fluid or the like into electronic form; to an image forming apparatus, which is a copy machine having a copy function of recording and printing the image converted into the electronic form by an image scanning apparatus; to an image forming apparatus, which is a multifunction peripheral having the copy function, to an image scanning method; and to a program which implements the aforementioned functionality.
2. Description of the Related Art
FIG. 1 is a block diagram showing an example of the configuration of an image forming apparatus, which has any of functions including copying, image scanning, image transmission, image reception, printing, box storage, and so on and which is a multifunction peripheral in a related art. The image forming apparatus in FIG. 1 includes an image scanning unit including an image sensor 101, a gain device 102, an analog-to-digital (A/D) converter 103, a shading corrector 104, and an image processor 106. The image forming apparatus typically also includes a user interface 109, a network controller 110, an image storage area 111, a controller 112, and a recording unit 113. The image forming apparatus in the related art reads an image to convert the read image into electronic form and to record the converted image. The operation of, particularly, the image scanning unit in the related art will be described with reference to FIG. 1.
In the image forming apparatus in FIG. 1, the image sensor 101 converts an image of a document into a voltage that is converted into an analog electrical signal. The analog signal obtains a predetermined amount of gain in the gain device 102. Generally, the voltage of the analog signal is set such that a desired white-paper or white reference value does not exceed a maximum value of the input voltage of the A/D converter 103 or such that the desired white-paper or white reference value is smaller than the maximum value of the input voltage of the A/D converter 103 by 10 to 20 percent. This is because, if the white-paper or white reference value is exceeded before the analog signal reaches the A/D converter 103, the information concerning the white-paper or white reference value is completely lost and, therefore, the information concerning high-luminance image data disappears. Decreasing the gain setting by more than 20 percent allows higher-luminance areas of a document to be digitized. However, in this case, the range of discrete information in the A/D converter 103 is expanded to degrade the precision of lower or intermediate luminances. As a result, it is not possible to yield a fine image.
For the above reason, an appropriate value is generally set in the gain device 102 such that the white-paper or white reference value is not exceeded and the tone in the lower or intermediate luminances is kept. The image information given an appropriate voltage in the gain device 102 is converted into a digital image in the A/D converter 103. The digital image is subjected to shading correction for every pixel in the shading corrector 104 according to Formula 1.V′n=C*Vn/Wn   [Formula 1]                where        V′n denotes the luminance of a digital image signal of the n-th pixel in a main scanning direction after the shading correction;        C denotes a shading correction coefficient;        Vn denotes the luminance of a digital image signal of the n-th pixel in the main scanning direction before the shading correction; and        Wn denotes the luminance of a digital luminance signal of the n-th pixel in the main scanning direction on a white reference plate (shading correction data (n denotes a position in the main scanning direction)).        
According to the above equation, the luminances of the pixels in the main scanning direction are corrected, and all the pixels are normalized so as to output a predetermined digital value and have the same output luminance for the reference white. At this time, the shading correction coefficient C, which is a coefficient of the luminance, is used to set a maximum output value, that is, an output level “255” in the case of 8-bit grayscale. Normally, the luminance level having the output level “255” is made slightly lower than a white-paper level and the white-paper level of the background is set so as to reach the value “255”. This is because reducing the luminance level of the background causes the background to become dark. The dark background has adverse effects including a decrease in compression ratio in compression of the information, a reduction in image quality in recording of the information, and a visual problem in display of the information.
FIGS. 2A to 2C illustrate adjustment of the luminance level from the gain adjustment to the shading correction in the image forming apparatus in the related art. FIGS. 2A to 2C show states of a typical output luminance when the image sensor 101 is included in a reduction optical system. As shown in FIG. 2A, the output luminance is curved upward at the center due to the difference in distance of light rays toward the device or a problem of the efficiency of the light emitting device. Next, as shown in FIG. 2B, the peak luminance at the center is given a gain to be set to a desired luminance by the gain adjustment and is digitized. Finally, as shown in FIG. 2C, all the pixels are normalized in the main scanning direction in the shading correction so as to provide a desired digital output.
Referring back to FIG. 1, the digital image normalized in the shading correction is subjected appropriate image processing in the image processor 106 in order to be transferred to a computer by copying, via facsimile communication, or over a network and to be displayed. In the image processor 106, the digital image is subjected to, for example, edge enhancement for finely displaying characters, change of the resolution, and compression. If the gain adjustment etc., described above, are automatically performed, the image processor 106 may control the gain device 102.
The digital image information is supplied to the image storage area 111, the recording unit 113, the network controller 110, and so on through a bus in the image forming apparatus and is subjected to desired processing.
The image scanning method on the basis of the white-paper or white reference value, described above, has a problem in that corrected trace with correction fluid etc. becomes completely invisible. In other words, since the correction fluid trace has a luminance value higher than that of the white paper in the image after the shading correction, the correction fluid trace has the same value (output level “255”), which is the maximum value, as the white paper.
FIGS. 5A and 5B show a correction fluid trace in images before and after the shading correction in the related art. As shown in FIG. 5B, the correction fluid trace is assimilated to the background and, therefore, persons other than a person who has read the document cannot detect whether the document has any correction fluid trace. In this case, although it is possible to yield a visually fine image, it is also possible to file a copy or an electric form of a receipt, financial statement, or public record that has been modified with the correction fluid and, thus, the reliability of the document is significantly decreased.
In addition, adjusting the shading correction coefficient so that the correction fluid trace can be detected in the document image that has been converted into electronic form causes the background not to have the output level “255” to substantially degrade the output image.
Under the situations described above, Japanese Patent Laid-Open No. 2001-236463 discloses a method of using two image scanners, that is, a reflective image scanner and a transparent image scanner, to yield a difference in luminance between the two image scanners in order to detect any correction fluid trace. In addition, Japanese Patent Laid-Open No. 2002-118735 discloses a method of comparing images before and after correction to detect and demonstrate any corrected portion.
However, in the method disclosed in Japanese Patent Laid-Open No. 2001-236463, it is inevitable to increases the apparatus in size and cost because of the use of the two types (reflective type and transparent type) of image scanners. In the method disclosed in the Japanese Patent Laid-Open No. 2002-118735, since it is necessary to read the image before correction in advance, it is difficult to ensure the reliability of the document that is tampered with the correction fluid.
Accordingly, there is a demand for an image scanning apparatus capable of detecting any correction fluid trace in a structure that is compact and reduced in cost while providing fine images similar to those in image scanning apparatuses in the related arts. There is also a demand for an image scanning apparatus capable of detecting whether a document converted into electronic form has any correction fluid trace. The image scanning apparatuses in the related arts have room for improvement in terms of these points.